a) Field of the Invention
This invention relates to a microscope and, more particularly, to a microscope in which high resolution microscopy of a sample is made by using radiation ranging from extreme vacuum ultraviolet light to soft X rays.
b) Description of the Prior Art
In general, a resolving power .delta. of the imaging optical system of a microscope is given by EQU .delta.=0.61.lambda./NA (1)
where .lambda. is the wavelength and NA is the numerical aperture.
In ordinary optical microscopes, when an oil-immersion objective lens of the largest numerical aperture of nearly 1.4 is used with visible light of a wavelength .lambda. of 5500 .ANG., the resolving power .delta., from Eq. (1), becomes 2400 .ANG. (0.24 .mu.m). Shortening of the wavelength .lambda. to be used is considered as a provision for improving the resolution in terms of Eq. (1). This brings about the development of ultraviolet microscopes using light in the ultraviolet region and X-ray microscopes using radiation in the soft X-ray region.
For the X-ray microscope, when the soft X rays having a wavelength of 40 .ANG. is applied, the resolving power .delta. amounts to 100 .ANG. (0.01 .mu.m) even with the use of an objective lens with a numerical aperture NA of 0.25. This is suitable for high resolution microscopy. In the soft X-ray region, however, the refractive index of any substance is nearly 1 and little reflection occurs, so that it is difficult to observe the reflected radiation from a sample. Thus, a transmission type microscope optical system had to be employed.
Referring now to FIG. 1, a description is given of such a conventional transmission type radiation microscope. In this microscope, a total reflection type mirror optical system utilizing a total reflection phenomenon is applied to the objective optical system. In the figure, reference numeral 1 denotes a laser plasma radiation source or a synchrotron radiation source; 2 a condenser composed of a grazing incidence type total reflection mirror, for focusing radiation rays emitted from the source 1 to irradiate a sample 3; 4 an objective composed of a grazing incidence type total reflection mirror, for imaging the X rays transmitted through the sample 3 on an image detector 5; and 6 a filter for blocking unwanted radiation. The image detector 5 employs an MCP (microchannel plate) or a solid-state image sensor, such as a CCD, CMD, AMI, etc., and an image detected by the detector 5 is displayed on a CRT not shown.
In the foregoing microscope, the grazing incidence type total reflection mirrors 2 and 4 can be substituted by zone plate optical systems using diffraction as well as normal incidence optical systems using multilayer film reflection mirrors. For the total reflection type mirror optical system or the normal incidence optical system, however, when a white radiation source, such as the synchrotron radiation source or the laser plasma radiation source, namely, a radiation source emitting not only X rays but also visible light and ultraviolet light is used, radiation excluding soft X rays used for observation will also reach the detector 5. This is because radiation of more than 500 .ANG. in wavelength length is transmitted through the optical system due to a metal reflection. Such unwanted radiation overlaps with the sensitivity region of the MCP or the CCD and causes deterioration of the detection accuracy and resolution of the image for observation in the detector 5. In order to remove the unwanted radiation, the filter 6 is used, made of beryllium or carbon constituting a thin film of 1000 to several thousand angstroms in thickness. Since soft X rays are absorbed into air, the microscope optical system shown in FIG. 1 is disposed in a vacuum.
As in the prior art mentioned above, it is difficult to observe the reflected radiation from the sample in the soft X-ray region, or to fabricate a beam splitter having good performance because of considerable absorption of soft X rays by material. The X-ray microscope thus has had to rely on a transmission type arrangement. Consequently, it has been impossible to observe a sample of large thickness.